Compression set
The compression set (DVR) is a measure of how elastomers behave in the event of long-term, constant compression set and subsequent relaxation.
Compression set in elastomers
In accordance with DIN 53 517 or DIN ISO 815 or ASTM D 395, the compression set (compression set) is measured with constant deformation. This represents the deformation proportion of the test material. Many test methods for elastomers , such as B. the tensile strength characterize the quality and consistency of the material. The compression set, on the other hand, is an important factor to consider before using a material for a particular purpose . The permanent deformation , the compression set (compression set), is an important parameter, especially for the use of seals and washers made of elastomers .
Test procedure
To determine this size, a cylindrical test specimen is placed around z. B. 25% compressed and stored at a certain temperature for a certain time. The temperature and the medium (mostly air, but also oils and other fluids) for the compression set test depends on the material to be tested, its intended use and the test setup (e.g. 24 h at 70 ° C for an EPDM profile in Building protection). 30 minutes after the load is removed , the height is measured again at room temperature and the permanent deformation is determined from this.
A different procedure (Daimler-Benz) stipulates that the test specimens cool down for 2 hours in the clamped state, then they are relaxed and measured after 30 minutes.
A compression set of 0% means that the body has fully regained its original thickness (impossible in reality), a compression set of 100% means that the body was completely deformed during the test and shows no recovery.
The calculation is based on the following formula: DVR (%) = (L0 - L2) / (L0 - L1) × 100%
In which:
- DVR = compression set in%
- L0 = height of the specimen before the test
- L1 = height of the specimen during the test (spacer)
- L2 = height of the test specimen after the test
Practical meaning
For example, a flange seal is compressed to a certain thickness during assembly and exerts a pressure on the flange surfaces. In the course of time, this pressure decreases as the rubber deforms plastically. If this plastic component - i.e. the compression set - is too large, the pressing force and thus the sealing effect decrease so much that leaks occur (these can occur over the entire surface but also in sections), especially if it is too "tight". The compression set, for flat seals , should not exceed a value of approx. 40% at continuous operating temperature.
Tensile set (ZVR)
In the case of O-rings or similar circular parts, the tensile set (ZVR) is determined in the same way : The diameter of the parts is determined with a tapered measuring mandrel , these are then clamped on a metal cylinder with a defined diameter and measured back after the load.
The calculation is based on the following formula: ZVR (%) = (H0 - H2) / (H0 - H1) × 100%
In which:
- ZVR = tensile set in%
- H0 = inner diameter of the ring before the test
- H1 = diameter of the test mandrel during loading
- H2 = inner diameter of the ring after the test
literature
- Gerhard P. Streit and 8 co-authors : Elastomere sealing systems, expert-Verlag, 2011, ISBN 978-3-8169-2895-9